Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Various ways of known of varying the speed ratio of a transmission. Some transmissions have a collection of gearing and shift elements configured such that engaging various subsets of the shift elements establish various power flow paths between an input shaft and an output shaft. These various power flow paths operate at different speed ratios between the input shaft and the output shaft. To change from one speed ratio to another speed ratio, one or more shift elements are disengaged and one or more shift elements are engaged in order to change which power flow path is utilized. Other transmissions utilize a variator to change speed ratio. A variator is capable of efficiently transmitting power at any speed ratio between an upper and lower limit and changing the speed ratio gradually while transmitting power. The upper and lower speed ratio limits of the variator may not match the speed ratio requirements of the vehicle. In that case, a transmission with a variator may also include gearing and shift elements such that the range of available speed ratios between the input shaft and the output shaft match vehicle requirements. The mechanism used to adjust the speed ratio influences the sensations experienced by vehicle occupants, including engine noise and vehicle acceleration.
FIG. 1 illustrates a front wheel drive (FWD) powertrain layout utilizing a Continuously Variable Transmission (CVT). Engine 10 converts chemical energy stored in liquid fuel into mechanical power to exert torque on a crankshaft. CVT 12 adapts the mechanical power from the crankshaft to exert torque on front wheels 14 and 16. Rear wheels 18 and 20 are not powered unless additional hardware is provided. CVT 12 includes several components including a launch device 22, a forward/reverse mechanism 24, a variator 26, and a differential 28. Launch device 22 permits transmission of torque even when the vehicle is stationary. Launch device 22 may be, for example, a torque converter or a launch clutch. Forward/reverse mechanism 24 establishes either a forward mode in which the engine power propels the vehicle forward or a reverse mode in which the direction of rotation and torque is reversed to propel the vehicle backwards. Variator 26 is controlled to establish various speed and torque ratios.
Several types of variator are known in the art. These variator types differ from one another in several respects including: range of ratio variability, torque transfer capacity, whether the input and output rotate in the same direction or the opposite direction, and whether the input and the output rotate about the same axis. A belt variator includes two adjustable sheaves, a driving sheave and a driven sheave, supported for rotation about two parallel axes. Each sheave may include two conical halves separated by a variable distance. A continuous belt with a relatively constant length and width frictionally engages both sheaves. As the two conical halves of a sheave are pushed together, the belt moves radially outward relative to the sheave's axis. Conversely, as the two conical halves of a sheave move apart, the belt moves radially inward relative to the sheave's axis. The belt transfers power from the driving sheave to the driven sheave at a speed ratio and torque ratio dictated by the radius of the frictional engagement point on each sheave. To increase the speed of the output relative to the input, the conical halves of the driving sheave are pushed closer together and the conical halves of the driven sheave are pushed apart. The radius of the frictional contact on the driving sheave increases while the radius of the frictional contact on the driven sheave decreases.